1. Field of the Invention
This invention relates to methods of printing, more specifically to printing using spatial light modulators.
2. Background of the Invention
The use of spatial light modulators in conjunction with a light source has many advantages over other types of optical printing, such as those employing scanned lasers. The spatial light modulator can use simpler illumination schemes, normally requires less peripheral equipment, and less power. Printing on large-area, low-sensitivity photosensitive materials, however, does bring up new areas of concern.
One of the many areas such photosensitive materials are used is in the patterning of printed circuit boards (PCBs), and printing plates. Normally, a sheet of such material or the negative that will be used to expose such media is wrapped around a drum, and the desired pattern is exposed onto the sheet using lasers while the drum spins, much like a xerographic printer. It would be an advantage to use spatial light modulators for reasons discussed above.
Some problems exist, however, with the use of spatial light modulators (SLMs), such as liquid crystal display cells (LCD), or deformable mirror devices (DMDs). In order for the machine to be cost-effective, it must produce a certain number of completed sheets of material in a given time frame. As it turns out, this requirement is difficult to meet using standard light sources and simple modulators. The light is not bright enough to expose the media within the time limit, as it is a "slow" media, requiring long exposure.
One solution is discussed in U.S. Pat. No. 5,049, 901. This solution uses a 1000 cell X 100 line spatial light modulator array. The data is loaded onto the cells of the array from the top down. After the first line of data is loaded onto the first row of cells, it is exposed onto the drum. Then the first line of data is then shifted down to the second row of cells. The second line of data is loaded onto the first row of cells, and then these two rows are exposed. The data shifting down the array is coordinated with the spinning of the drum, so the same data is exposed onto the same line on the drum for approximately 100 lines.
Typically, the illumination patterns from conventional light sources are brighter in the center than at the edges, and no illumination pattern is completely uniform. This is corrected with the last lines of the array. Depending on the amount of correction necessary, the number of lines is up to the designer. The center cell or cells are turned off after a predetermined number of lines. The cells on either side of the center region are left on for a pre-determined number of lines. The cells further away from the center regions are left on for even more lines. This continues until the last line, in which only the cells at the edges remain on. In this way, the darker areas of the image are exposed for longer durations to equalize exposure time across the image.
This solution involves the use of a modulator array consisting of an array of shift registers rather than conventional x/y addressing. The registers shift the data down the array, as discussed previously. For some applications, or some modulators, shift registers are impractical because of the amount of space they require. In the case of the DMD, the complexity of the shift registers makes it difficult to manufacture the device using the already-established processing techniques.